The necessity of protecting structural steel such as columns, beams, girders and other steel assemblies from the damaging effect of fire is an important part of modern building design. Steel does not burn, but can lose strength at high temperatures. As a result, a variety of fire protection systems have been developed to insulate steel from the effects of fire in order to prolong the time required for steel to reach a temperature of about 538° C., generally by at least two hours, depending upon local fire regulations.
Intumescent coatings are coatings that react under the influence of heat and swell to 10-100 times their original thickness, producing an insulating char that protects the substrate to which the coating is applied from the effects of fire. Due to the fact that intumescent coatings are applied at a relatively low thickness, as compared with the thickness required for other types of insulating materials to achieve a similar fire protection rating, they are increasingly becoming the preferred choice for structural fire protection. Another attractive feature of intumescent coatings is their smooth and aesthetically pleasing finish. Thin film intumescent coatings therefore allow architects and designers to maximize the creative design possibilities of structural steel.
Typical intumescent coatings usually comprise a minimum of four components: a source of mineral acid catalyst, typically ammonium polyphosphate; a source of carbon, typically pentaerythritol or dipentaerythritol; a blowing agent, typically melamine; and a binder, typically a thermoplastic resin. When an intumescent coating is subjected to heat, a series of reactions occur. The ammonium polyphosphate decomposes to produce polyphosphoric acid, catalyzing the dehydration of pentaerythritol to produce char. The blowing agent also starts to decompose, giving off non-flammable gases that cause the carbon char to foam, thus producing a meringue-like structure that is highly effective in insulating the substrate from heat. The basic function of the binder is to bind together the components of the intumescent coating, so that they may be applied to the substrate and held in intimate contact therewith until required to perform their function in a fire situation. Furthermore, the binder contributes to the formation of a uniform cellular foam structure, since the molten binder helps trap the gases given of by the decomposing blowing agents, thus ensuring a controlled expansion of the char.
Intumescent coatings are generally categorized into three types: water based, solvent based, and epoxy based. Water-based and solvent-based intumescent coatings are among the most widely used products (over 80% usage in the North American market). These coatings utilize a thermoplastic binder, such as polyvinyl chloride (PVC), polyurethane, polyester, polyvinyl acetate, phenolic resin or acrylic resin. The thermoplastic characteristics of the binder allow the coating to swell significantly (with blowing agent) and form chars 10-100 times the original coating thickness. Therefore, only a relatively thin film is required with water or solvent based coatings. However, a significant drawback of these types of coatings is the time associated with installation. Depending on the coating thickness required for fireproofing, a project could last from 2 days to over one week, since only a limited thickness (usually 40-50 mils or 1.0-1.2 mm per day) can be sprayed in a single application without sagging or peeling. The coating must be allowed to dry before a second layer can be applied, prolonging the overall installation time. Environmental conditions, such as humidity, can affect the drying time of the coating. In addition, a trained applicator must apply the coating to ensure that a uniform thickness is applied. For solvent-based systems, the applicator must be aware of special safety considerations, for example inhalation hazards and flammability. Finally, sprayed on coatings are messy and necessitate extensive cleanup of the job site following installation. In order to solve some or all of these problems in the art, improved fire protection barriers are needed.
Epoxy-based coatings (e.g. PPG's Pitt-Char® and Akzo Nobel's Chartek® systems) have great durability and are mostly used for outdoor applications, such as offshore platforms or industrial plants. Because of the thermosetting nature of epoxy resins, epoxy-based coatings swell poorly upon heating (only a few times their original thickness) and consequently require greater amounts to be applied in order to attain the desired fire protection rating. The cost of epoxy systems is usually much higher than water-based and solvent-based systems, meaning that the overall project cost is prohibitive for interior applications. In addition, the aesthetic finish is compromised due to the much greater coating thickness required.
Coatings are often reinforced using, for example, short length pieces of fiberglass mixed with the coating during application. The random direction of the fibers mixed throughout the coating lends reinforcement, reducing the likelihood of sagging, and allowing greater overall coating thickness to be applied to increase fire protection ratings beyond what can be achieved without reinforcement. However, the use of fiberglass reinforcement is messy and does not mitigate the other disadvantages of sprayed on coatings.
Fiberglass insulating batons impregnated with a form of carbon called graphite (another intumescent material) are used as wraps in certain fire protection applications. These wraps do not generally comprise a continuous adhesive layer along the face being affixed to the substrate. The wraps can occasionally employ an adhesive strip in order to adhere a portion of the wrap to itself; however, the wrap then only remains in contact with the substrate due to friction. The lack of intimate contact between the wrap and the material being protected from fire means that, upon charring, the intumescent material has an increased likelihood of prematurely detaching from the substrate, which compromises fire protection.
When an intumescent material is applied around corners or to a rounded exterior surface (such as to a hollow tube or around a structural I-beam), fissures can develop upon expansion of the material during a fire. These fissures can propagate all of the way through to the substrate, thereby leading to premature exposure of the material in a fire situation. It would therefore be desirable to reduce the likelihood of fissure propagation through to the substrate material.
U.S. Pat. No. 5,851,663 (Parsons, et al.) discloses a pressure sensitive adhesive composition that includes an intumescent material intermingled therewith. The intumescent material is added to increase fire resistance of the tape itself, rather than to act as a fire protection barrier for the substrate it is adhered to. No multi-layer fire protection barrier is disclosed that comprises separate layers of intumescent material and adhesive. In addition, the maximum reported expansion of the composition is 7.5 times, which is generally considered insufficient for use in fire barrier applications.
U.S. Pat. No. 6,866,928 (Kobe, et al.) and US Patent Publication 2003/0175497 (Fischer, et al.) both describe fire retardant tapes comprising a stretchable release layer. These tapes do not comprise a layer of an intumescent material and exhibit little or no expansion during a fire. These tapes are therefore not suitable for use as intumescent fire protection barriers.
Korean Patent Publication 2002034134 (Cho, J. Y.) discloses a thermally expanding fire retardant tape comprising a thin steel plate with a plurality of slits therethrough that is coated with a synthetic rubber composition consisting of an olefinic polymer mixed with a fire retardant material. The fire retardant material is therefore not provided in a separate layer. The steel plate also impedes flexibility of the tape and increases its weight, making it difficult to apply as a fire protection barrier.
U.S. Pat. No. 5,681,640 (Kiser) discloses a fire protection barrier comprising folded layers of a metallic fire resistant material and an intumescent material. The layers are designed to unfold during a fire to permit expansion of the intumescent material. The fire protection barrier may be attached to a substrate using a strip of adhesive tape. No porous continuous reinforcing matrix is disclosed. Due to its folded nature, this barrier is not suitable for sequential application in multiple layers.
U.S. Pat. No. 4,058,643 (Marshall, et al.) describes a fire protection barrier comprising a fiberglass insulation material adhesively bonded to a plastic sheath. The adhesive comprises an intumescent material that expands during a fire to prevent the sheath from melting and wicking into the fiberglass insulation. There are no separate intumescent and adhesive layers and no adhesive attachment to the substrate.
A need therefore still exists for improved intumescent fire protection barriers comprising an adhesive layer for attachment of the barrier to a substrate.